1. Field of the Invention
The present invention relates to a camera and, more specifically, to an exposure controlling apparatus of a camera having a plurality of areas for measuring brightness.
2. Description of the Related Art
In an exposure controlling apparatus of a camera having a plurality of areas for measuring brightness, a method for measuring brightness has been proposed in which a suitable controlling brightness value is provided by calculating weighted mean of measured brightness values of respective areas. Coefficients (weight coefficients) multiplied by the respective measured brightness values include zero but not negative numbers. Therefore, the controlling brightness value will be an intermediate value between the maximum and the minimum values of the measured brightness values of the areas.
It is proposed in Japanese Patent Laying-Open Gazette No. 279829/1986 that a plurality of arithmetic expressions for weighted mean should be prepared in advance and the expression employed is switched in accordance with the photographing magnification in the above described method for measuring brightness.
In an exposure controlling apparatus of a camera having a plurality of areas for measuring brightness, how to determine the controlling brightness value for controlling exposure from the brightness values measured at the plurality of areas in order to provide best exposure is an important concern. In the above mentioned Japanese Patent Laying-Open Gazette No. 279829/1986, weighted mean equations for calculating the controlling brightness value from respective measured brightness values are switched in accordance with the photographing magnification so as to properly control the exposure, on the understanding that the optimal controlling brightness value can be provided by proper weighted mean of respective measured brightness values.
However, according to an analysis of various photographs with the photographing conditions changed variously, that is, with the subject distance, the focal length of the taking lens, the photographing magnification changed variously for different distribution of brightness, the optimal controlling brightness value can not always be calculated by the weighted mean of respective measured brightness values. The optimal controlling brightness value is often provided by applying to the measured brightness value an exposure correction which is determined independent from the absolute value of the measured brightness. Especially when a subject is to be photographed with rear light, with the brightness at the peripheral portion brighter than the brightness of a central portion by about +1.5 to 2.5 EV, the best controlling brightness value may not exist between the maximum value and the minimum value of the measured brightness obtained at respective areas for measuring brightness. In such cases, optimal exposure controlling can not be realized in the prior art.
When there is a big difference between the brightness of a main subject at the center and the brightness of the background at the periphery, as is often the case of photographs with rear light, if the controlling brightness value for controlling the exposure is determined by averaged brightness measuring, center-weighted measuring, or other measuring employing arbitrary weighted mean, then the controlling brightness value always becomes higher than the measured brightness value of the main subject, whose brightness is low. Consequently, the exposure for the main subject tends to be insufficient and the main subject will be dark as a shadow. Namely, proper exposure for the main subject can not be provided. In view of the foregoing, it is recommended that the low brightness of the main subject is employed as the control brightness value for controlling exposure by spot metering of a small area at the center in taking photographs with rear light.
Similarly, in taking a photograph in accordance with so called shadow reference exposure in which the brightness of a dark portion in the scene to be photographed is measured by spot metering and the measured value is used as the black level, when a white subject exists near the area for the spot metering, the result of the spot metering does not exactly correspond to the brightness of the dark portion but comprises an error in the brighter side. The inventors of the present invention studied also employing statistical analysis how the errors in measuring brightness change under various conditions. Consequently, it is found by the inventor that the error in spot metering is determined based on prescribed conditions which has relation neither with the absolute value of the spot measured brightness value nor with the absolute value of the measured brightness of the peripheral portions.